My feelings are mixed on this one. I not a real fan of difficult games, I never play a game for the challenge but the story, the experience and the world it immerses me in. That said I'm not for super-easy gaming, and I think boss fights should be tough, especially the final boss. You really can't beat that feeling of euphoria when you take down the last boss in a game.
However I do have one gripe - game over screens and in particular ones that send you back to your last save. I see no real purpose for this, its as if the game is punishing me (lol) when I'm forced to lose hours of progress. Sure you can shore up against this by saving often, but why should I have to protect myself from a deliberate game mechanic? My reaction to the 'game over' screen is the same as that if my computer crashed and my progress was lost - anger and frustration.
I'd opt instead for being kicked out of the area or having to restart the fight - but not losing your progress i.e. levels, gear etc. I also think the problem of difficulty is so easily solvable by introducing difficulty options which should be the norm whenever possible.